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An ion implantation system at LAAS
technological facility in Toulouse, France.

Ion implantation is a materials engineering process by which ions of a material can be implanted into
another solid, thereby changing the physical properties of the
solid. Ion implantation is used in semiconductor device fabrication and in
metal finishing, as well as various applications in materials
science research. The ions introduce both a chemical change in
the target, in that they can be a different element than the
target, and a structural change, in that the crystal
structure of the target can be damaged or even destroyed by the
energetic collision cascades.

General
principle

Ion implantation setup with mass separator

Ion implantation equipment typically consists of an ion source, where ions of
the desired element are produced, an accelerator, where the ions are
electrostatically accelerated to a high energy, and a target
chamber, where the ions impinge on a target, which is the material
to be implanted. Thus ion implantation is a special case of particle
radiation. Each ion is typically a single atom or molecule, and
thus the actual amount of material implanted in the target is the
integral over time of the ion current. This amount is called the
dose. The currents supplied by implanters are typically small
(microamperes), and thus the dose which can be implanted in a
reasonable amount of time is small. Thus, ion implantation finds
application in cases where the amount of chemical change required
is small.

Typical ion energies are in the range of 10 to 500 keV (1,600 to 80,000 aJ). Energies in the
range 1 to 10 keV (160 to 1,600 aJ) can be used, but result in a
penetration of only a few nanometers or less. Energies lower than
this result in very little damage to the target, and fall under the
designation ion beam deposition. Higher
energies can also be used: accelerators capable of 5 MeV (800,000
aJ) are common. However, there is often great structural damage to
the target, and because the depth distribution is broad, the net
composition change at any point in the target will be small.

The energy of the ions, as well as the ion species and the
composition of the target determine the depth of penetration of the
ions in the solid: A monoenergetic ion beam will generally have a
broad depth distribution. The average penetration depth is called
the range of the ions. Under typical circumstances ion ranges will
be between 10 nanometers and 1 micrometer. Thus, ion implantation
is especially useful in cases where the chemical or structural
change is desired to be near the surface of the target. Ions
gradually lose their energy as they travel through the solid, both
from occasional collisions with target atoms (which cause abrupt
energy transfers) and from a mild drag from overlap of electron
orbitals, which is a continuous process. The loss of ion energy in
the target is called stopping.

The introduction of dopants in a semiconductor is the most
common application of ion implantation. Dopant ions such as boron,
phosphorus or arsenic are generally created from a gas source, so
that the purity of the source can be very high. These gases tend to
be very hazardous. When implanted in a semiconductor, each dopant
atom creates a charge carrier in the semiconductor (hole or
electron, depending on if it is a p-type or n-type dopant), thus
modifying the conductivity of the semiconductor in its
vicinity.

One prominent method for preparing silicon on insulator (SOI)
substrates from conventional silicon substrates is the SIMOX
(Separation by IMplantation of
OXygen) process, wherein a buried high dose oxygen
implant is converted to silicon oxide by a high temperature annealing process.

Mesotaxy

Mesotaxy is the term for the growth of a crystallographically
matching phase underneath the surface of the host crystal (compare
to epitaxy, which is the
growth of the matching phase on the surface of a substrate). In
this process, ions are implanted at a high enough energy and dose
into a material to create a layer of a second phase, and the
temperature is controlled so that the crystal structure of the
target is not destroyed. The crystal orientation of the layer can
be engineered to match that of the target, even though the exact
crystal structure and lattice constant may be very different. For
example, after the implantation of nickel ions into a silicon
wafer, a layer of nickel silicide can be grown in which the crystal
orientation of the silicide matches that of the silicon.

Application in metal
finishing

Tool steel
toughening

Nitrogen or other ions can be implanted into a tool steel target
(drill bits, for example). The structural change caused by the
implantation produces a surface compression in the steel, which
prevents crack propagation and thus makes the material more
resistant to fracture. The chemical change can also make the tool
more resistant to corrosion.

Surface
finishing

In some applications, for example prosthetic devices such as
artificial joints, it is desired to have surfaces very resistant to
both chemical corrosion and wear due to friction. Ion implantation
is used in such cases to engineer the surfaces of such devices for
more reliable performance. As in the case of tool steels, the
surface modification caused by ion implantation includes both a
surface compression which prevents crack propagation and an
alloying of the surface to make it more chemically resistant to
corrosion.

Problems with ion
implantation

Crystallographic damage

Each individual ion produces many point defects in the target
crystal on impact such as vacancies and interstitials. Vacancies
are crystal lattice points unoccupied by an atom: in this case the
ion collides with a target atom, resulting in transfer of a
significant amount of energy to the target atom such that it leaves
its crystal site. This target atom then itself becomes a projectile
in the solid, and can cause successive collision events.
Interstitials result when such atoms (or the original ion itself)
come to rest in the solid, but find no vacant space in the lattice
to reside. These point defects can migrate and cluster with each
other, resulting in dislocation loops and other defects.

Damage
recovery

Because ion implantation causes damage to the crystal structure
of the target which is often unwanted, ion implantation processing
is often followed by a thermal annealing. This can be referred to
as damage recovery.

Amorphization

The amount of crystallographic damage can be enough to
completely amorphize the surface of the target: i.e. it can become
an amorphous
solid (such a solid produced from a melt is called a glass). In some cases, complete
amorphization of a target is preferable to a highly defective
crystal: An amorphized film can be regrown at a lower temperature
than required to anneal a highly damaged crystal.

Sputtering

Some of the collision events result in atoms being ejected (sputtered) from the
surface, and thus ion implantation will slowly etch away a surface.
The effect is only appreciable for very large doses.

Ion
channelling

If there is a crystallographic structure to the target, and
especially in semiconductor substrates where the crystal structure
is more open, particular crystallographic directions offer much
lower stopping than other directions. The result is that the range
of an ion can be much longer if the ion travels exactly along a
particular direction, for example the <110> direction in silicon and other diamond cubic
materials. This effect is called ion channelling, and,
like all the channelling effects, is highly
nonlinear, with small variations from perfect orientation resulting
in extreme differences in implantation depth. For this reason, most
implantation is carried out a few degrees off-axis, where tiny
alignment errors will have more predictable effects. There is no
relation between this effect and ion channel of a cell membrane.

Ion channelling can be used directly in Rutherford backscattering and related
techniques as an analytical method to determine the amount and
depth profile of damage in crystalline thin film materials.

Hazardous Materials Note

In the ion implantation semiconductor fabrication process of wafers,
it is important for the workers to minimize their exposure to the
toxic
materials used in the ion implanter process. Such hazardous
elements, solid source and gasses are used, such as Arsine and Phosphine. For this reason, the semiconductor fabrication facilities are
highly automated, and may feature negative pressure gas bottles
safe delivery system (SDS). Other elements may include Antimony, Arsenic, Phosphorus, and Boron. Residue of these elements show up when the
machine is opened to atmosphere, and can also be accumulated and
found concentrated in the vacuum pumps hardware. It is important
not to expose yourself to these carcinogenic, corrosive, flammable, and toxic elements. Many
overlapping safety protocols must be used when handling these
deadly compounds. Use safety, and read MSDS's.

High Voltage
Safety

High voltage power supplies in ion implantation equipment can
pose a risk of electrocution. In addition, high-energy atomic
collisions can, in some cases, generate radionuclides. Operators and Maintenance
personnel should learn and follow the safety advice of the
manufacturer and/or the institution responsible for the equipment.
Prior to entry to high voltage area, terminal components must be
grounded using a grounding stick. Next, power supplies should be
locked in the off state and tagged to prevent unauthorized
energizing.